1. Field of the Invention
The present invention relates to an optical module for transmitting, receiving or transmitting/receiving plural optical signals.
2. Description of the Related Art
At distal ends of optical communication systems, an optical module is used for transmitting/receiving optical signals converted from electrical signals to/from optical transmission lines, respectively, such as optical fibers. Particularly, an optical module is used that has a PD array or an LD array packaged therein to transmit or receive plural optical signals.
FIG. 15 is a plan view for explaining a reception optical module that receives respective optical signals from plural optical fibers with plural light-receiving elements (e.g., photodiodes: PDs) respectively.
As shown in FIG. 15, an optical module 120 comprises an optical fiber array 122 having plural optical fibers 121, a PD array 124 having plural light-receiving elements (PDs) 123 for respectively receiving respective optical signals from the optical fibers 121 of the optical fiber array 122, and a lens component 125 interposed between the PD array 124 and the optical fiber array 122 for discretely guiding the optical signals between the optical fibers 121 of the optical fiber array 122 and the corresponding PDs 123 of the PD array 124. The lens component 125 is provided with plural optical fiber side convex lenses 126 and plural PD side convex lenses 127.
The respective optical signals emitted from the optical fibers 121 are collimated by the optical fiber side convex lenses 126 respectively, so that each collimated light (parallel light) is collected by the PD side convex lenses 127 respectively, and received by the corresponding PDs 123 substantially at focal position. In this optical module, because one set of the convex lenses (micro lenses) is provided for each optical fiber and PD corresponding thereto, the respective optical signals from the optical fibers 121 can be discretely coupled to the PDs 123 (See JP-A-2005-31556).
The lens component 125 packaged in the conventional optical module 120 uses the plural micro lenses arranged parallel to each other to discretely collimate the plural optical signals. However, for example, because of a specified spacing between the PDs 123 of the PD array 124, in the case of a small spacing (pitch) between the optical fibers 121 or the PDs 123, the diameter (effective diameter or aperture) of the convex lenses has to be small. Also, the NA (No. of apertures) of the optical fibers 121 is predetermined, and light (optical signals) are emitted therefrom with a predetermined expansion angle. Thus to inject light emitted from the optical fibers 121 into a small diameter lens without loss, i.e., to inject thereinto light when the beam diameter of the emitted light is smaller than the diameter of the lens), the LDs or PDs 123 have to be provided right close to the PD side convex lenses 127 of the lens component 125.
However, for the structural reason of LD or PD packages with LDs or PDs, there is the problem that they cannot be arranged within the focal distance of the lenses. That is, there is the problem that LDs or PDs cannot be arranged in a desired position in the optical module, regardless of structure of LD or PD packages.
Also, generally, convex lenses that are slightly contaminated with dust degrade optical signals transmitted therethrough. In the conventional optical module 120, because the diameter of the convex lenses is small and the size ratio of contamination to lens diameter (optical signal diameter) is therefore large, there is the problem that contamination adhering to lens surface degrades optical signals.
As shown in FIG. 16, in collecting plural (e.g., four) optical signals to plural optical fibers 132 arranged at a specified interval, when the plural optical signals are injected into one lens 131 having a diameter that allows them to be injected thereinto, the respective optical axes of the optical signals passed through the lens 131 are not parallel to each other, to be injected obliquely into the plural optical fibers 132 respectively arranged parallel to each other, which therefore causes coupling loss.